Gas detection devices

ABSTRACT

Gas detector includes structure (10) defining a gas flow passage (11), means (12) for passing an ultraviolet light beam (13) across the passage, and first and second electrodes (16, 17) downstream of the ultraviolet light means (12), at least one of the first and second electrodes (16, 17) being in the form of a series of electrically separate segments (16a, 17a) extending in a downstream direction, the electrodes (16, 17) being connectable to potential applying means (18) and to means (20) whereby the current passing through each segment (16a, 17a) of a segmented electrode (16, 17) can be measured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to gas detection devices, particularlythose of the type which use the ionisation of gases by ultra-violetlight. One use of such devices is in the detection and investigation ofgas flows, when a tracer gas is introduced into a main gas flow. Anotheris in the detection of the presence of an extraneous gas in a flow of aparticular gas.

2. Discussion of Prior Art

The main gas flow with which these devices is used is of air, and thisspecification will therefore, for convenience, refer to the main gasflow as airflow. Similarly, for convenience, any other gas in theairflow, whether a tracer gas or an extraneous gas, will be referred toas a test gas.

Devices of this type operate by passing the airflow through anultra-violet light from a lamp which is preferably tuned for maximumionisation effect on the test gas, and measuring the current flowbetween two electrodes caused by ionisation of the gas as the airflow ispassed between the electrodes. Such devices are described in, forexample, GB 1576474 and PCT/GB92/01313. In the latter of these anionising ultra-violet light is passed through a gas-carrying airstreamwhich then passes between two co-axial potentially biased annularelectrodes, ionisation being substantially completed before the airflowenters the gap between the electrodes. The device is calibrated so thatthe current between the electrodes provides a measure of theconcentration of the test gas in the airstream.

In the device of PCT/GB92/01313 the airflow therethrough is driven by afan. However, the device is intended to be used in the open air, wherethe airspeed may be affected by local wind effects. This can affect theresults. It is therefore desirable that the airspeed through the devicebe known. Conventional airspeed measuring devices (i.e. pivot tubes,anemometers) tend to the disproportionately expensive or complicated,and also in some cases may disrupt the measurement process.

SUMMARY OF THE INVENTION

According to the present invention a gas detector includes structuredefining a gas flow passage, means for passing an ultraviolet light beamacross the passage, and first and second electrodes downstream of theultra-violet light means, at least one of the first and secondelectrodes being in the form of a series of electrically separatesegments extending in a downstream direction, the electrodes beingconnectable to potential applying means and to means whereby the currentpassing through each segment of a segmented electrode can be measured.

In use air containing a concentration of ionisable test gas is passedthrough the air passage and subjected to the beam of ultra-violet lightwhich causes a degree of ionisation in the test gas. Potential isapplied across the electrodes, which causes ions to be collected by theelectrodes, the resultant currents being measured and the measurementspassed to analysing means which compare the readings from consecutivesegments to provide a measurement of the air speed through the passageand which also sum the segment readings to provide the concentration ofthe test gas. The measurement of air speed may be displayed, and willusually be used directly by the analysing means.

There may be only one segmented electrode, in which case the potentialapplied between the unsegmented electrode and the segments will be thesame in each case, or both electrodes may be segmented, in which casedifferent potentials can be applied between associated pairs ofelectrodes.

In one form of the invention the gas passage is rectangular and thesegments are in the form of flat rectangular strips. In another form thegas passage is circular, the electrodes are in co-axial form as inPCT/GB92/01313, and the segments are in annular form, in the form ofsections of a rod, or both.

The detector may be provided with means, such as a fan, for inducingairflow through the passage, or may rely on flow speed (for example windspeed, or air speed in a duct) to induce flow, in which case electrodesmay be provided on each side of the ultra-violet light source to allowfor flow in either direction through the passage.

It will be realised, of course, that measurement of velocity through thedetector by analysing readings from consecutive segments of theelectrodes will require prior calibration of the detector.

According to another aspect of the present invention a method ofmeasuring the velocity of flow, of an airflow containing a test gas,through a detector of the type wherein the flow is passed through anUltra Violet (UV) light beam and then between two potentially biassedelectrodes, at least one of the electrodes being in the form of a seriesof electrically separate segments extending in a downstream direction,includes the steps of calibrating the detector, of passing the airflowthrough the UV light beam, and of comparing readings of currents causedby ions falling on consecutive segments.

Yet another aspect of the present invention might dispense with the needfor calibration.

According to yet another aspect of the present invention a method ofmeasuring the velocity of flow, of an airflow containing a test gas,through a detector of the type wherein the flow is passed through anUltra Violet (UV) light beam and then between two potentially biassedelectrodes, at least one of the electrodes being in the form of a seriesof electrically separate segments extending in a downstream direction, asubsequent current across the electrodes being measured, includes thestep of pulsing the UV light beam and noting a time interval between apulse and response to the pulse as indicated by a sudden change in thecurrent measurement caused by ions being collected by at least one ofthe segments.

In a conventional gas detector operating on the photo-ionisationprinciple, and ultra-violet radiation source is continuously maintainedso as to produce a constant intensity and uninterrupted source ofultra-violet radiation to which the gas stream is exposed. The ultimatesensitivity of trace gas detection depends on the number of ions of thetrace gas that can be created and collected by the combined system ofultra-violet radiation source and electrode collector means describedabove. At very low trace gas concentrations the ion current due to theionised portion of the trace gas may become indistinguishable from thesignal noise level in the system, and this determines the lower limit ofdetection of the device. Since only a small proportion of the availabletrace gas is in practice ionised, the number of ions produced can beincreased in this operating range simply by increasing the power of theultra-violet source so that a more intense flow of ultra violetradiation is produced, thereby producing more ions of the tracer gas,and thus increasing the lower limit of detection of the device. However,the simple expedient of increasing the power of the ultra-violet sourcewhilst maintaining it in continuous emission has certain disadvantagessuch as:

i) the operating life of the ultra-violet source is likely to besignificantly reduced by operating it continuously at higher power, and

ii) for sources of the type in which ultra violet radiation passes via atransparent window into the gas stream, the problem of fouling of thewindow by products of the ionisation process is likely to be made worse,and

iii) the power consumption of the device will be increased.

A feature of the pulsing process is that, as an addition to being usedas a velocity measuring means, it can be used to provide enhanced lowerlimit sensitivity of trace gas detection, whilst overcoming to someextent the above problems.

In one such embodiment of the invention, an ultra-violet radiationsource is provided by a gas discharge maintained in a cell by thepassage of an electric current between electrodes embodied in the cell,or by the provision of a radio-frequency exciter circuit the inductiveelement of which surrounds the cell. Means are provided for turning thedischarge on and off, or for causing the discharge to switch between twostates of discharge distinguished by their differing power levels, atsome specified frequency. During the high power segments of thedischarge cycle, a high power pulse stream of ultra-violet radiation isproduced, and this impinging on the trace gas in the airstream produceshigher numbers of ions of the trace gas than at lower power levels.

The lower limit sensitivity of the device to the trace gas concentrationis thus increased, approximately in proportion to the peak power levelenhancement of the ultra violet radiation intensity over its equivalentcontinuous emission level.

The pulsating system also provides secondary advantages in that

i) the overall power and duty cycle combination of the ultra-violetradiation source need not be significantly altered from its continuousmode characteristic, so that there need be no significant change in theuseful lifetime of the ultra violet source,

ii) the duty cycle of the ion production corresponds to a similarexposure of the source window to the products of ionisation as for thecontinuous mode of operation, so that fouling of the source windowshould not be worsened, and

iii) the overall power requirements of the pulsed ultra-violet sourceneed not be significantly greater than in its continuous-mode ofoperation.

In case of fluctuating concentrations the variation in concentration canof course be recovered from the output signal by the appropriate signalprocessing techniques.

The detector according to the present invention may be used, if desired,in conjunction with ultra-violet measuring means which measure the lossof ultra-violet from the beam crossing the gas flow passage. Such meansare described in out co-pending Application PCT GB 93/02333 now assignedU.S. Ser. No. 08/436,273, filed on May 16, 1995, and now abandoned.

The electrodes may be positioned after a bend in the gas flow passagedownstream of the ultra-violet light source as described in ourco-pending Application GB 9226663.

BRIEF DESCRIPTION OF THE FIGURES

Some embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying diagrammatic drawings,of which:

FIG. 1 is an elevation, in section along line I--I of FIG. 2, of a gasdetector according to the invention,

FIG. 2 is an end view, in the direction of arrow A in FIG. 1,

FIG. 3 is a plan view of part of the detector of FIG. 1,

FIG. 4 is an elevation, in section along line IV--IV of FIG. 5, ofanother embodiment of the invention, and

FIG. 5 is an end view in the direction of arrow B in FIG. 4.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

A gas detector (FIGS. 1 to 3) has structure 10 defining a rectangulargas flow passage 11. An ultra-violet lamp 12 is positioned to send abeam 13 of ultra-violet light across the passage 11. An ultra-violetlight loss measuring device 14, as described in PCT/GB 93/02333 may bepositioned to collect light crossing the passage 11. A fan 15 ispositioned at one end of the passage 11 to induce flow in the directionof arrow F through the passage 11, and downstream of the lamp 12 are twoopposing electrodes 16, 17. The electrodes 16, 17 are divided into aplurality of paired segments 16a , 17a.

In use, each pair of electrodes is connected to a potential applyingdevice (such as a battery) 18 in a circuit 19 which includes a currentmeasuring device 20 whose readings are passed to a computer 21. The fan15 is operated to induce flow of air through the passage 11 and the lamp12 is operated to send a beam of light 13 across the flow. If the flowof air contains an ionisable test gas this is ionized by the beam 13 adthe resultant ions are collected by the electrodes 16, 17 causingreadings to be taken by the current measuring devices 20. The readingsof devices 20 from consecutive pairings of segments 16a, 17a, arecompared by the computer 21 with calibrations to give the speed of flowthrough the passage 11 and the readings from all devices 20 are summedby the computer 21 and compared with calibrations (in which speed offlow will usually be a factor) to give the concentration of test gas inthe flow.

Calibration is carried out by passing an airflow containing a test gasat various speeds through the flow passage 11, operating the UV lamp 18,applying a potential across the paired segments 16a, 17a of theelectrodes 16, 17, and noting the current reading. The calibration cannot conveniently be stored in the computer 21,

For use in a pulsed mode the UV lamp can be powered by a supply 118including power means 119 and a switch 120, the switch being operated bymeans controlled by the computer 21.

In a more complicated form of UV supply, an ultra-violet radiationsource is provided by a gas discharge maintained in a cell by thepassage of an electric current between electrodes embodied in the cell,or by the provision of a radio-frequency exciter circuit the inductiveelement of which surrounds the cell. The computer 21 controls means forturning the discharge on and off, or for causing the discharge to switchbetween two states of discharge distinguished by their differing powerlevels, at some specified frequency. During the high power segments ofthe discharge cycle, a high power pulse stream of ultra-violet radiationis produced, and this impinging on the trace gas in the airstreamproduces higher numbers of ions of the trace gas than at lower powerlevels.

For use with an airstream containing a single trace gas it will usuallybe unnecessary to calibrate the apparatus in the pulsed mode.

In use in the pulsed mode the time between a pulse and the change inreading due to ions being collected by a least one of the segments 16a,17a is measured.

An alternative version of the device is adapted to cope with flow ineither direction through the passage 11. The fan 15 is omitted, andelectrodes 16', 17', which are mirror images of the electrodes 16, 17are installed on the opposite side of the lamp 12 to the electrodes 16,17.

One of the electrodes 16, 17 may be left unsegmented. In this case thepotential between this electrode and each segment of the other electrodewill be the same.

In another embodiment of the invention (FIGS. 4 and 5) the configurationis the same other than that the gas flow passage 11 is of cylindricalsection and the electrodes 26, 27 are co-axial, as in out co-pendingPCT/GB92/01313. With this embodiment the central electrode, shown as 26,will usually be in the form of a rod and will usually be unsegmented.Constructional considerations are such that whilst an embodiment havinga segmented central electrode is not impossible it is somewhatcomplicated. Segments 27a of the electrode 27 are of annular form.

Further embodiments within the scope of the invention will be apparentto those skilled in the art. For example unidirectional forms of theinvention may have no fan therein, relying on flow speed such as windspeed or the speed of flow in a duct to induce flow therethrough.

Using the pulsed flow method it might be possible to calculate theconcentrations of more than one test gas in an airstream by adjustingthe rate of flow through the passage 13 such that ions of particulartrace gases are collected substantially on different segments of theelectrodes 16 17.

Further details of the operation of this type of gas detector are givenin our various co-pending Applications mentioned above and will not berepeated here.

What is claimed is:
 1. A method of measuring the velocity of flow, of anairflow containing a test gas, through a detector of the type whereinthe flow is passed through an Ultra Violet (UV) light bean and thenbetween two potentially biased electrodes, wherein at least one of theelectrodes is in the form of a series of electrically separate segmentsextending in a downstream direction, said method including the stepsof:calibrating the detector passing the airflow through the UV lightbeam, comparing readings of currents caused by ions falling onconsecutive segment to calibration currents and providing said flowvelocity based upon said comparison.
 2. A method as claimed in claim 1further including the step of introducing airflow through a passage ofthe detector.
 3. A method as claimed in claim 2 wherein said inducingstep uses a fan.
 4. A method as claimed in claim 3 wherein said inducingstep includes using the fan to control the speed of flow through thepassage.
 5. A method as claimed in claim 1 further including the step ofusing air flow speed to induce flow through a passage of the detector.6. A method as claimed in claim 5 are used on each side of anultra-violet light source for permitting flow in either directionthrough the passage.
 7. A method as claimed in claim 1 wherein thedetector is used in conjunction with ultra-violet measuring means whichmeasure the loss of ultra-violet from the beam crossing the gas flowpassage.
 8. A method as claimed in claim 1 and further including thesteps of:summing the readings of currents caused by ions falling on allsegments; and comparing said sum of the readings with calibration datato provide a measure of concentration of test gas in the airflow.